1. Field of the Invention
This invention pertains generally to methods and devices for sensing angular positions and, more particularly, to methods and devices for detecting and providing feedback of the angular position of a rotary control valve.
2. Description of the Prior Art
Rotary control valves, such as butterfly valves or ball valves, include a valve body and a plate, ball, or other flow control member rotatably mounted in the valve body to either block fluid flow through the valve, or allow fluid flow through the valve, depending upon the angular position of the flow control member. For example, a ball valve includes a ball which is securely mounted to upper and lower portions of a valve shaft. The ball is mounted in the fluid flow path of the valve by mounting the lower portion of the valve shaft in a lower portion of the valve body and the upper portion of the valve shaft in an upper portion of the valve body, with the ball positioned between the upper and lower shaft portions in the fluid flow path of the valve. An actuator is attached to the upper portion of the valve shaft, which extends through the valve body. When the actuator is turned, the valve shaft, and, therefore, the valve ball, is rotated. The ball is shaped, i.e., portions of the ball are removed or grooves are formed therein, such that when the ball is rotated, through, e.g., 90.degree., the fluid flow path through the valve is gradually opened or closed.
It is often desirable to determine accurately the angular position of the ball within the ball valve, and therefore, the state, i.e., open, closed, or somewhere in between, of the valve. Several methods of automatically determining the angular position of a valve are known. Sensing the angular position of the valve may be accomplished by attaching an angular position sensor to the valve actuator. For example, magnets may be attached to the rotating member of the valve actuator, and a Hall effect sensor used to determine the position of the actuator as the actuator member, therefore, the magnets attached thereto, is rotated. As the actuator member is rotated, the magnetic field produced by the magnets attached to the actuator is also rotated. The Hall sensor is placed within the magnetic field produced by the magnets. As the direction of the magnetic field changes, as the actuator is rotated, the Hall effect sensor detects the change and provides a signal from which the rotary position of the actuator can be determined.
Alternatively, a cam may be attached to the actuator shaft. The angular position of the actuator shaft is then converted to an electrical signal by an inductive sensor connected or in close proximity to the cam. As the actuator is rotated, the cam attached thereto is also rotated, which, in turn, changes the inductance of the inductive sensor in contact with or in close proximity to the cam. Thus, a signal is provided by the inductive sensor which is related to the angular position of the actuator and from which the angular position of the actuator can be determined.
As a third alternative, a potentiometer may be connected to the rotating member of the valve actuator. As the actuator member is rotated, the potentiometer potential is changed. This change in potential can be detected and signal derived therefrom from which the angular position of the actuator can be determined.
All known methods for determining the angular position of a valve by mounting a rotary position sensor on the valve actuator, however, suffers from a serious limitation. For a ball valve, for example, an accurate determination of the rotary position of the valve ball is desired. Although the valve actuator is connected, via the valve shaft, to the valve ball, there could be some inherent looseness in this connection. Even if the connection between the valve actuator and the ball is initially tight, this connection can fail or become looser with time. Thus, sensing the angular position of the valve actuator will not necessarily translate into an accurate indication of the position of the valve flow control member.
In order to determine the angular position of the flow control member more accurately, the angular position sensing methods described above have been employed to sense the position of the lower portion of the valve shaft which is directly connected to the flow control member. Since the shaft is directly and tightly connected to the flow control member, sensing the angular position of the shaft will result in an accurate determination of the angular position of the flow control member itself. Any of the angular position sensing methods described above may be used to determine the angular position of the valve shaft. For example, a magnet may be attached to the lower portion of the valve shaft, and a Hall sensor placed near the magnet. As the valve shaft, and, therefore, the flow control member itself, rotates, the magnetic field produced by the magnet attached to the valve shaft changes direction. This change in direction is detected by the Hall effect sensor, which provides a signal related to the angular position of the angular shaft member from which the angular position of the flow control member can be determined accurately.
Alternatively, a potentiometer can be attached to the lower portion of the valve shaft. As the shaft, and, therefore, the flow control member itself, is rotated, the potential of the potentiometer is changed. This change can be sensed, and a signal provided from which the angular position of the flow control member can be determined accurately.
Although measuring the angular position of a flow control member by sensing the angular position of the lower portion of the valve shaft can achieve accurate results, known methods for making such measurements suffer from other limitations. In order to measure the movement of the lower portion of the valve shaft, with a potentiometer or another device, the lower portion of the valve shaft must be extended through the bottom of the valve, and the potentiometer or other measurement device attached to the shaft on the outside of the valve. Extending the lower portion of the valve shaft thus provides another leak path from the valve, and the added packing adds friction to the valve. Also, extending the lower portion of the shaft through the valve body makes the valve more fragile during moving and handling of the valve.
As an alternative to extending the lower portion of the valve shaft through the valve body, the potentiometer or other device for sensing angular position of the shaft may be extended through an aperture in the valve body near the end of the shaft. For example, a potentiometer may be mounted on the outside of the valve body. An elongated shaft attached to the potentiometer may be extended through an aperture in the valve body wall and be connected to the lower portion of the valve shaft. Alternatively, a Hall effect device may be mounted within the valve body, near a magnet placed on the flow control member or lower portion of the valve shaft, with conducting wires for conducting the signal provided by the Hall effect sensor passing through a hole in the valve body. In either case, the addition of another aperture to the valve body provides another potential leak path from the valve, and therefore, adversely affects valve integrity.
Another limitation of Hall effect and other magnetic field sensors employed to detect the angular position of the lower shaft of a rotary control valve is the effect of temperature changes on the accuracy of such devices. Changes in temperature of the magnet mounted on the lower portion of the valve shaft and the magnetic field sensing device itself can affect the signal provided by the sensor. Temperature changes, can, therefore, affect the accuracy of the angular position sensed by such a detector unless temperature compensation is provided.